Visual Studio macro: Find files that aren’t included in the project?

Here is the C# version of your code:

public static void IncludeNewFiles()
{
    int count = 0;
    EnvDTE80.DTE2 dte2;
    List<string> newfiles;

    dte2 = (EnvDTE80.DTE2)System.Runtime.InteropServices.Marshal.GetActiveObject("VisualStudio.DTE.10.0");

    foreach (Project project in dte2.Solution.Projects)
    {
        if (project.UniqueName.EndsWith(".csproj"))
        {
            newfiles = GetFilesNotInProject(project);

            foreach (var file in newfiles)
                project.ProjectItems.AddFromFile(file);

            count += newfiles.Count;
        }
    }
    dte2.StatusBar.Text = String.Format("{0} new file{1} included in the project.", count, (count == 1 ? "" : "s"));
}

public static List<string> GetAllProjectFiles(ProjectItems projectItems, string extension)
{
    List<string> returnValue = new List<string>();

    foreach(ProjectItem projectItem in projectItems)
    {
        for (short i = 1; i <= projectItems.Count; i++)
        {
            string fileName = projectItem.FileNames[i];
            if (Path.GetExtension(fileName).ToLower() == extension)
                returnValue.Add(fileName);
        }
        returnValue.AddRange(GetAllProjectFiles(projectItem.ProjectItems, extension));        
    }

    return returnValue;
}

public static List<string> GetFilesNotInProject(Project project)
{
    List<string> returnValue = new List<string>();
    string startPath = Path.GetDirectoryName(project.FullName);
    List<string> projectFiles = GetAllProjectFiles(project.ProjectItems, ".cs");

    foreach (var file in Directory.GetFiles(startPath, "*.cs", SearchOption.AllDirectories))
        if (!projectFiles.Contains(file)) returnValue.Add(file);

    return returnValue;
}